Steel material for high-pressure hydrogen gas environment, steel structure for high-pressure hydrogen gas environment, and methods for producing steel material for high-pressure hydrogen gas environment

ABSTRACT

A steel material and methods for producing the same. The steel material exhibits excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment and is, therefore, suitable for use in hydrogen storage tanks, hydrogen line pipes, and the like. The steel material has a specified chemical composition, a tensile strength of 560 MPa or higher, and a fracture toughness value K IH  exhibited by the steel material in a high-pressure hydrogen gas atmosphere is 40 MPa·m 1/2  or higher.

TECHNICAL FIELD

This application relates to a steel material and a steel structure thatare suitable for use in a high-pressure hydrogen gas environment and tomethods for producing a steel material for a high-pressure hydrogen gasenvironment. In particular, the application relates to improvement inhydrogen embrittlement resistance exhibited by a steel material in ahigh-pressure hydrogen gas environment.

BACKGROUND

Hydrogen has been attracting significant worldwide attention in recentyears because hydrogen can be a clean energy source, and diversificationof energy is being considered. In particular, high expectations areplaced on fuel cell vehicles that use high-pressure hydrogen gas as afuel source. Accordingly, the development of fuel cell vehicles is beingwidely advanced worldwide, and some fuel cell vehicles have been putinto practical use.

Fuel cell vehicles run on hydrogen stored in a tank instead of gasoline,which is conventionally used. Accordingly, in place of gasolinestations, hydrogen stations where fuel replenishment can be performedare necessary. For achieving widespread use of fuel cell vehicles, it isimportant to build, in ordinary urban districts, a large number ofhydrogen stations where fuel replenishment can be performed.

In hydrogen stations, typically, a vehicle-mounted hydrogen fuel tank isdirectly filled with hydrogen from a hydrogen storage tank that storeshydrogen at high pressure, by using a pressure differential method. Itis assumed that a target pressure for filling a vehicle-mounted hydrogenfuel tank is on the order of 70 MPa so that a range comparable to thatof gasoline-powered vehicles can be achieved. Accordingly, it is assumedthat the pressure of the storage tank of a hydrogen station needs to behigher, that is, on the order of 82 MPa. Hence, it is required thatstorage tanks of hydrogen stations be able to store and supply hydrogensafely in a high-pressure hydrogen gas environment.

Furthermore, instances in which transportation of large amounts ofhydrogen gas is carried out by utilizing a pipeline are anticipated. Insuch instances, the transport pressure is on the order of 10 MPa, and,therefore, the line pipes are exposed to a hydrogen gas pressure on theorder of 10 MPa.

Thus, steel structures for hydrogen, such as storage tanks of hydrogenstations, which store and supply high-pressure hydrogen gas, and linepipes that are utilized for mass transportation of hydrogen gas, areused while being exposed to a high-pressure hydrogen gas environment.

One possible material for steel structures is a low-alloy steelmaterial, which has the advantages of being inexpensive and having highstrength. However, low-alloy steel materials have the problem ofbecoming brittle when hydrogen is absorbed into the alloys, that is, theproblem of susceptibility to a so-called “hydrogen embrittlement”.

Accordingly, in the related art, austenitic stainless steel, such asSUS316L, which is less susceptible to hydrogen embrittlement thanlow-alloy steels, has been utilized in steel structures that are used ina high-pressure hydrogen gas environment. However, austenitic stainlesssteel, such as SUS316L, comes with a high cost of the steel materialand, in addition, has low strength; therefore, in cases where austeniticstainless steel is designed to withstand a high hydrogen pressure, thesteel has a large thickness, and the price of the resulting structurefor hydrogen is increased. Accordingly, there has been a strong needfor, as a material for steel structures for hydrogen, a low-alloy steelmaterial that is less expensive and can withstand a high-pressurehydrogen gas environment.

To address this need, Patent Literature 1, for example, proposes a steelfor a high-pressure hydrogen environment. The steel for a high-pressurehydrogen environment described in Patent Literature 1 is a steel that isused in a high-pressure hydrogen environment, the steel having achemical composition containing, in mass %, C: 0.03 to 0.18%, Si: 0.1 to0.5%, Mn: 0.2 to 1.8%, P: 0.025% or less, S: 0.002 to 0.02%, sol. Al:0.01 to 0.10%, and Ca: 0.001 to 0.10% and optionally further containingV: 0.03 to 0.3%, wherein Ca/S: less than 1.5 or 11 or more, with thebalance being Fe and incidental impurities. According to the technologydescribed in Patent Literature 1, MnS or Ca complex inclusions and VC,which serve as trap sites for diffusible hydrogen, are formed to convertdiffusible hydrogen to non-diffusible hydrogen, thereby reducing adiffusible hydrogen concentration ratio to inhibit embrittlement due todiffusible hydrogen.

Furthermore, Patent Literature 2 proposes a high-strength low-alloysteel excellent in high-pressure hydrogen environment embrittlementresistance characteristics. The high-strength low-alloy steel describedin Patent Literature 2 is a high-strength steel having a compositioncontaining, in mass %, C: 0.10 to 0.20%, Si: 0.10 to 0.40%, Mn: 0.50 to1.20%, P: 0.005% or less, S: 0.005% or less, Cr: 0.20 to 0.80%, Cu: 0.10to 0.50%, Mo: 0.10 to 1.00%, V: 0.01 to 0.10%, B: 0.0005 to 0.005%, andN: 0.01% or less, with the balance being Fe and incidental impurities.According to Patent Literature 2, it is preferable that quenching beperformed at 920° C. or higher, and subsequently, a tempering process beperformed at a relatively high temperature within a range of 600 to 640°C. to adjust a tensile strength to be in a very narrow range of 900 to950 MPa. Patent Literature 2 states that, as a result, the high-strengthlow-alloy steel excellent in high-pressure hydrogen environmentembrittlement resistance characteristics exhibits excellent elongationand drawing characteristics even in a 45 MPa hydrogen atmosphere.

Furthermore, Patent Literature 3 proposes a high-strength low-alloysteel excellent in high-pressure hydrogen environment embrittlementresistance characteristics. The high-strength low-alloy steel describedin Patent Literature 3 is a Cr—Mo high-strength low-alloy steel having acomposition containing, in mass %, C: 0.10 to 0.20%, Si: 0.10 to 0.40%,Mn: 0.50 to 1.20%, P: 0.005% or less, S: 0.002% or less, Ni: 0.75 to1.75%, Cr: 0.20 to 0.80%, Cu: 0.10 to 0.50%, Mo: 0.10 to 1.00%, V: 0.01to 0.10%, B: 0.0005 to 0.005%, and N: 0.01% or less and furthercontaining one or both of Nb: 0.01 to 0.10% and Ti: 0.005 to 0.050%,with the balance being Fe and incidental impurities. According to PatentLiterature 3, it is preferable that normalizing be performed at 1000 to1100° C., quenching be performed from a temperature range of 880 to 900°C., and thereafter, a tempering process be performed at a relativelyhigh temperature of 560 to 580° C., to ensure that a grain size afterthermal refining is 8.4 or higher in terms of a grain size number andadjust a tensile strength to be in a very narrow range of 900 to 950MPa. Patent Literature 3 states that, as a result, the high-strengthlow-alloy steel excellent in high-pressure hydrogen environmentembrittlement resistance characteristics exhibits excellent elongationand drawing characteristics even in a 45 MPa hydrogen atmosphere.

Furthermore, Patent Literature 4 proposes a low-alloy steel for ahigh-pressure hydrogen gas environment. The low-alloy steel described inPatent Literature 4 is a low-alloy steel for a high-pressure hydrogengas environment and has a composition containing, in mass %, C: 0.15 to0.60%, Si: 0.05 to 0.5%, Mn: 0.05 to 3.0%, P: 0.025% or less, S: 0.010%or less, Al: 0.005 to 0.10%, Mo: 0.5 to 3.0%, V: 0.05 to 0.30%, O(oxygen): 0.01% or less, and N: 0.03% or less, with the balance being Feand incidental impurities, the low-alloy steel having a tensile strengthof 900 MPa or higher. Note that Patent Literature 4 states that thecomposition described above may additionally contain B: 0.0003 to0.003%. Patent Literature 4 states that, in this instance, it ispreferable that the N content be adjusted to 0.010% or less. PatentLiterature 4 states that, in the technology described therein, V is tobe added, a Mo content is to be increased compared with existing steels,a tempering temperature is to be increased, and V—Mo carbides are to beutilized; as a result, a carbide morphology at grain boundaries isimproved, and, consequently, hydrogen environment embrittlementresistance is significantly improved.

Furthermore, Patent Literature 5 proposes a steel for a high-pressurehydrogen gas storage container, the steel having excellent hydrogenresistance. The steel for a high-pressure hydrogen gas storage containerdescribed in Patent Literature 5 is a steel having a compositioncontaining, in mass %, C: 0.12 to 0.15%, Si: 0.01 to 0.10%, Mn: 0.30 to0.60%, P: 0.02% or less, S: 0.005% or less, Cr: 2.00 to 2.50%, Mo: 0.90to 1.20%, V: 0.20 to 0.35%, Nb: 0.01 to 0.06%, and Ti: 0.002 to 0.030%,with the balance being Fe and incidental impurities, wherein an MCcarbide precipitation index MCI=(0.24V+0.06Mo)/C of 0.70 or higher issatisfied. Patent Literature 5 states that, in the technology describedtherein, during the production of a steel plate, stress relief annealingthat lasts for a long time is to be performed on a steel having theabove-described composition after a normalizing process: as a result, MCcarbides (Mo, V)C are dispersively precipitated in a refined and denseform, and, consequently, hydrogen resistance exhibited by the steel,such as hydrogen embrittlement resistance, is improved.

Furthermore, Patent Literature 6 proposes a steel material for storageof high-pressure hydrogen. The steel material described in PatentLiterature 6 is a steel material containing, in mass %, C: 0.05 to0.12%, Si: 0.01 to 0.50%, Mn: more than 0.6 to 1.8%, P: 0.02% or less,S: 0.003% or less, and Al: 0.01 to 0.08%, with the balance being Fe andincidental impurities, the steel material having a metallurgicalstructure formed primarily of bainite, which is present in an areafraction of 90% or higher, with cementite being dispersivelyprecipitated in the bainite, the cementite having an average particlediameter of 50 nm or less and an average aspect ratio of 3 or less.Patent Literature 6 states that, in the technology described therein,fine cementite having a low aspect ratio is to be dispersed; as aresult, an amount of hydrogen absorption from a high-pressure hydrogenatmosphere is reduced, the toughness of the base metal is improved, and,consequently, embrittlement due to hydrogen is inhibited.

Furthermore, Patent Literature 7 proposes a high-strength steel materialfor a high-pressure hydrogen storage container. The high-strength steelmaterial described in Patent Literature 7 is a high-strength steelmaterial containing, in mass %, C: 0.05 to 0.15%, Si: 0.01 to 0.50%, Mn:more than 0.6 to 2.5%, P: 0.02% or less, S: 0.003% or less, and Al: 0.01to 0.08%, wherein Pcm is 0.19 or higher, with the balance being Fe andincidental impurities, the steel material having a metallurgicalstructure including lower bainite, which is present in an area fractionof 70% or more, and a martensite-austenite constituent, which is presentin an area fraction of 3% or less, and the steel material having atensile strength of 780 MPa or higher. Patent Literature 7 states that,in the technology described therein, a lower bainite structure is to beemployed, cementite is to be finely precipitated, and the formation ofcoarse cementite and a martensite-austenite constituent is to beinhibited; as a result, hydrogen absorption is inhibited, and,consequently, embrittlement in a high-pressure hydrogen environment anda decrease in ductility are prevented.

Furthermore, Patent Literature 8 describes a steel material havingexcellent fatigue crack propagation resistance for a high-pressurehydrogen environment. The steel material described in Patent Literature8 is a steel material having a chemical composition containing, in mass%, C: 0.05 to 0.60%, Si: 0.01 to 2.0%, Mn: 0.3 to 3.0%, P: 0.001 to0.040%, S: 0.0001 to 0.010%, N: 0.0001 to 0.0060%, and Al: 0.01 to 1.5%,the chemical composition further containing one or more of Ti: 0.01 to0.20%, Nb: 0.01 to 0.20%, and V: 0.01% or more and less than 0.05%, andthe chemical composition further containing one or more of B: 0.0001 to0.01%, Mo: 0.005 to 2.0%, and Cr: 0.005 to 3.0%; the steel materialhaving a microstructure in which tempered martensite is present in avolume fraction of 95% or higher, precipitates including at least one ofTi, Nb, and V and at least one of carbon and nitrogen and having adiameter of 100 nm or less are present in a density of 50 particles/μm²or higher, and a prior austenite grain diameter is 3 μm or larger.Patent Literature 8 states that, with the technology described therein,fatigue crack propagation speed in a high-pressure hydrogen environmentwith a pressure of 80 MPa or higher is dramatically reduced comparedwith existing steels; therefore, the service life of hydrogen storagetanks and the like that are used in a high-pressure hydrogen environmentis improved, and the safety of hydrogen storage containers that are usedin a high-pressure hydrogen environment is improved.

Note that Non Patent Literature 1 and Non Patent Literature 2 provide afracture toughness value of a low-alloy steel.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2005-2386

PTL 2: Japanese Unexamined Patent Application Publication No. 2009-46737

PTL 3: Japanese Unexamined Patent Application Publication No.2009-275249

PTL 4: Japanese Unexamined Patent Application Publication No. 2009-74122

PTL 5: Japanese Unexamined Patent Application Publication No. 2010-37655

PTL 6: Japanese Unexamined Patent Application Publication No.2012-107332

PTL 7: Japanese Unexamined Patent Application Publication No.2012-107333

PTL 8: Japanese Patent No. 5633664

Non Patent Literature

NPL 1: MATSUMOTO Takuya et al., Transactions of the Japan Society ofMechanical Engineers Series A, Vol. 79, No. 804 (2013), pp. 1210-1225

NPL 2: MATSUOKA Saburo et al., M&M 2016 Zairyo Rikigaku Conference(Strength of Materials Conference), OS16-10, (2016), pp. 813-815

SUMMARY Technical Problem

In particular, in steel structures that are used in a high-pressurehydrogen gas environment, such as hydrogen storage tanks, hydrogenfilling is repeatedly performed, and, therefore, the structures(containers) experience cyclic stresses. Accordingly, the designing of asteel structure such as a hydrogen storage tank requires taking fatiguefailure into consideration. It is said that a critical point of fatiguefailure of a steel structure that is used in a high-pressure hydrogengas environment is related to a fracture toughness value K_(IH)exhibited by a steel material in hydrogen gas. It is assumed that oneeffective approach for extending the life of steel structures forhydrogen and improving the safety thereof is to increase the fracturetoughness value K_(IH) exhibited by a steel material in hydrogen gas.

For increasing the fracture toughness value K_(IH) exhibited by a steelmaterial in hydrogen gas, a suitable approach is, for example, to reduceupper bainite, which contains coarse carbides.

Unfortunately, a problem has been encountered in that, with therelated-art technologies described above, the fracture toughness valueK_(IH) exhibited by a steel material in hydrogen gas cannot besufficiently increased.

The disclosed embodiments have made in view of the problem with therelated-art technologies described above, and objects of the disclosedembodiments are to provide a steel material that exhibits excellenthydrogen embrittlement resistance in a high-pressure hydrogen gasenvironment, the steel material being suitable for use in a steelstructure that is used in a high-pressure hydrogen gas environment,examples of the steel structure including hydrogen storage tanks andhydrogen line pipes, to provide a steel structure, and to providemethods for producing a steel material for a high-pressure hydrogen gasenvironment.

Note that as used herein, the expression “exhibit excellent hydrogenembrittlement resistance in a high-pressure hydrogen gas environment”refers to instances in which the fracture toughness value K_(IH) is 40MPa·m^(1/2) or higher as determined by conducting a fracture toughnesstest in a hydrogen gas atmosphere at room temperature (20±10° C.) and apressure of 115 MPa in accordance with The Japan Pressure containerResearch Council, Division of Materials Science and Technology, HydrogenGas Embrittlement Technical Committee, Task Group V (1991).

Note that when the fracture toughness value K_(IH) is 40 MPa·m^(1/2) orhigher, it is possible to design a steel structure for hydrogen,examples of which include storage tanks and line pipes, in which LBB(Leak Before Break) can be established, in a manner such that athickness range that can be used in a process for producing a steelpipe, such as a seamless steel pipe or a UOE pipe, is ensured.

Furthermore, as used herein, the term “steel material” encompasses steelsheets, steel plates, seamless steel pipes, electric resistance weldedsteel pipes, shaped steels, steel bars, and the like.

Solution to Problem

To achieve the objects described above, the inventor diligentlyperformed studies regarding various factors that affect the hydrogenembrittlement resistance of carbon steel materials and low-alloy steelmaterials. As a result, it was newly discovered that in cases where Si,Cu, and Al, each in an amount of 0.5% or more in mass %, are included incombination, with Al being optional, the hydrogen embrittlementresistance exhibited by the steel material in a high-pressure hydrogengas atmosphere is noticeably improved. Furthermore, it was discoveredthat in cases where a steel material is cooled uniformly so as to maketimings of transformation uniform, a local temperature increase due tolocalization of heat generation involved in the transformation isinhibited, and, consequently, the formation of martensite or lowerbainite is facilitated, which results in an increase in the fracturetoughness value K_(IH) exhibited by a steel material in hydrogen gas.

The disclosed embodiments were completed based on the above findings andwith additional studies. Specifically, a summary of the disclosedembodiments is as follows.

(1) A steel material for a high-pressure hydrogen gas environment whichhas a composition containing, in mass %, C: 0.04 to 0.50%, Si: 0.5 to2.0%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.010% or less, N: 0.0005 to0.0080%, Al: 0.010% to 2.0%, O: 0.0100% or less, and Cu: 0.5 to 2.0%,with the balance being Fe and incidental impurities, wherein the steelmaterial for a high-pressure hydrogen gas environment has a tensilestrength of 560 MPa or higher, and a fracture toughness value K_(IH)exhibited by the steel material in a high-pressure hydrogen gasatmosphere is 40 MPa·m^(1/2) or higher.(2) The steel material for a high-pressure hydrogen gas environmentaccording to (1), wherein the composition contains, in mass %, Al: 0.5to 2.0%.(3) The steel material for a high-pressure hydrogen gas environmentaccording to (1) or (2), wherein the composition additionally contains,in mass %, one or more selected from Ni: 0.05 to 2.00%, Cr: 0.10 to2.50%, Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V:0.005 to 0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050%.(4) The steel material for a high-pressure hydrogen gas environmentaccording to any one of (1) to (3), wherein the composition additionallycontains, in mass %, one or more selected from Nd: 0.005 to 1.000%, Ca:0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0050%.(5) A steel structure for a high-pressure hydrogen gas environment whichcomprises the steel material for a high-pressure hydrogen gasenvironment according to any one of (1) to (4).(6) The steel structure for a high-pressure hydrogen gas environmentaccording to (5), wherein the steel structure is a storage tank or aline pipe.(7) A method for producing a steel material for a high-pressure hydrogengas environment which comprises heating a steel starting material havinga composition containing, in mass %, C: 0.04 to 0.50%, Si: 0.5 to 2.0%,Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.010% or less, N: 0.0005 to0.0080%, Al: 0.010% to 2.0%, O: 0.0100% or less, and Cu: 0.5 to 2.0%,with the balance being Fe and incidental impurities, to a temperature ofAc₃ transformation temperature or higher and then hot-rolling the steelstarting material to form a steel material having a predetermined shape;and thereafter subjecting the steel material to an accelerated coolingprocess, in which the steel material is cooled from a temperature of(Ar₃ transformation temperature—50° C.) or higher to a cooling stoptemperature of 600° C. or less at a cooling rate of 1 to 200° C./s,wherein the steel material for a high-pressure hydrogen gas environmenthas a tensile strength of 560 MPa or higher, and a fracture toughnessvalue K_(IH) exhibited by the steel material in a high-pressure hydrogengas atmosphere is 40 MPa·m^(1/2) or higher.(8) A method for producing a steel material for a high-pressure hydrogengas environment which comprises heating a steel starting material havinga composition containing, in mass %, C: 0.04 to 0.50%, Si: 0.5 to 2.0%,Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.010% or less, N: 0.0005 to0.0080%, Al: 0.010% to 2.0%, O: 0.0100% or less, and Cu: 0.5 to 2.0%,with the balance being Fe and incidental impurities, to a temperature ofAc₃ transformation temperature or higher and then hot-rolling the steelstarting material to form a steel material having a predetermined shape;and thereafter subjecting the steel material to a directquenching-tempering process, in which the steel material is cooled froma temperature of (Ar₃ transformation temperature—50° C.) or higher to acooling stop temperature of 250° C. or lower at a cooling rate of 1 to200° C./s, and further, the steel material is tempered at a temperatureof Ac₁ transformation temperature or lower, wherein the steel materialfor a high-pressure hydrogen gas environment has a tensile strength of560 MPa or higher, and a fracture toughness value K_(IH) exhibited bythe steel material in a high-pressure hydrogen gas atmosphere is 40MPa·m^(1/2) or higher.(9) A method for producing a steel material for a high-pressure hydrogengas environment which comprises subjecting a steel material having acomposition containing, in mass %, C: 0.04 to 0.50%, Si: 0.5 to 2.0%,Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.010% or less, N: 0.0005 to0.0080%, Al: 0.010% to 2.0%, O: 0.0100% or less, and Cu: 0.5 to 2.0%,with the balance being Fe and incidental impurities, the steel materialbeing formed to have a predetermined shape, to areheating-quenching-tempering process, in which the steel material isheated to a temperature of Ac₃ transformation temperature or higher, thesteel material is subsequently subjected to water quenching or oilquenching, and further, the steel material is tempered at a temperatureof Ac₁ transformation temperature or lower, wherein the steel materialfor a high-pressure hydrogen gas environment has a tensile strength of560 MPa or higher, and a fracture toughness value K_(IH) exhibited bythe steel material in a high-pressure hydrogen gas atmosphere is 40MPa·m^(1/2) or higher.(10) The method for producing a steel material for a high-pressurehydrogen gas environment according to any one of (7) to (9), wherein thecomposition contains, in mass %, Al: 0.5 to 2.0%.(11) The method for producing a steel material for a high-pressurehydrogen gas environment according to any one of (7) to (10), whereinthe composition additionally contains, in mass %, one or more selectedfrom Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%, Mo: 0.05 to 2.00%, W: 0.05 to2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.200%, Ti: 0.005 to 0.100%, andB: 0.0005 to 0.0050%.(12) The method for producing a steel material for a high-pressurehydrogen gas environment according to any one of (7) to (11), whereinthe composition additionally contains, in mass %, one or more selectedfrom Nd: 0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%,and REM: 0.0005 to 0.0050%.

Advantageous Effects

With the disclosed embodiments, a steel material having significantlyimproved hydrogen embrittlement resistance exhibited in a high-pressurehydrogen gas environment can be easily and conveniently produced, and,therefore, a remarkable industrial effect is provided. Furthermore, withthe disclosed embodiments, the following effect is also provided: thehydrogen embrittlement resistance of steel structures such as storagetanks for high-pressure hydrogen gas and line pipes for high-pressurehydrogen gas is noticeably improved, and, therefore, the fatigueresistance thereof is improved; as a result, a significant contributionis made to the extension of the life of steel structures.

DETAILED DESCRIPTION

A steel material of the disclosed embodiments has, as a basiccomposition, a composition containing, in mass %, C: 0.04 to 0.50%, Si:0.5 to 2.0%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.010% or less, N:0.0005 to 0.0080%, Al: 0.010% to 2.0%, O: 0.0100% or less, and Cu: 0.5to 2.0%, with the balance being Fe and incidental impurities.

First, reasons for the limitations on the composition of the steelmaterial of the disclosed embodiments will be described. Note that inthe following description, “mass %” in the context of a composition willbe denoted simply as “%”.

Studies performed by the inventor discovered that when a material isdeformed in a hydrogen gas, Si, Cu, and Al, with Al being optional,enable the resulting dislocation to have a vein structure, therebyproducing an effect of increasing the fracture toughness value K_(IH)exhibited in hydrogen gas. Accordingly, hydrogen embrittlementresistance is improved. This effect is noticeable at least in instancesin which Si and Cu are included each in an amount of 0.5% or more, andthe effect is more noticeable in instances in which Al is optionallyincluded in an amount of 0.5% or more. Hence, in the disclosedembodiments, Si: 0.5 to 2.0% and Cu: 0.5 to 2.0% are included, and,optionally, Al: 0.5 to 2.0% is included.

Si: 0.5 to 2.0%

Similar to Cu and Al, Si is an element that improves hydrogenembrittlement resistance. In the disclosed embodiments, Si is to bepresent in an amount more than or equal to 0.5%. On the other hand, if alarge amount of Si is present, that is, an amount more than 2.0%, thegrain boundaries become brittle, and, therefore, toughness is degraded.Accordingly, the amount of Si is limited to a range of 0.5 to 2.0%. Notethat the amount is preferably more than or equal to 0.75% and less thanor equal to 2.00%. More preferably, the amount is more than or equal to1.00%.

Cu: 0.5 to 2.0%

Similar to Si and Al, Cu is an element that improves hydrogenembrittlement resistance. In the disclosed embodiments, Cu is to bepresent in an amount more than or equal to 0.5%. On the other hand, if alarge amount of Cu is present, that is, an amount more than 2.0%,susceptibility to hot cracking, which may occur during heating orwelding, is increased. Accordingly, the amount of Cu is limited to arange of 0.5 to 2.0%. Note that the amount is preferably more than orequal to 0.75% and less than or equal to 2.00%. More preferably, theamount is more than or equal to 1.00%.

Al: 0.010% to 2.0%

Similar to Si and Cu, Al is an element that contributes to improvinghydrogen embrittlement resistance. Even when being present in arelatively small amount, Al acts as a deoxidizer and forms a nitride AlNto inhibit the coarsening of grains, which may occur during heating,thereby contributing to refining a microstructure. To produce theseeffects, the amount of Al is specified to be more than or equal to0.010% in the disclosed embodiments. On the other hand, if a largeamount of Al is present, that is, an amount more than 2.0%,susceptibility to surface defects in the steel material is increased.Note that in terms of noticeably improving hydrogen embrittlementresistance, it is preferable that the amount of Al be more than or equalto 0.5% and less than or equal to 2.0%. More preferably, the amount ismore than or equal to 0.75%, and even more preferably more than or equalto 1.00%.

Note that reasons for the limitations on the components other than Si,Cu, or Al are as follows.

C: 0.04 to 0.50%

C is an element that contributes to increasing strength and improveshardenability. C needs to be present in an amount more than or equal to0.04% so that a desired strength and hardenability can be ensured. Onthe other hand, if C is present in an amount more than 0.50%,weldability is significantly degraded, and the toughness of the basemetal and a weld heat affected zone is degraded. Accordingly, the amountof C is limited to a range of 0.04 to 0.50%. Note that the amount ispreferably more than or equal to 0.10% and less than or equal to 0.45%.

Mn: 0.5 to 2.0%

Mn is an element that contributes to increasing strength by improvinghardenability. Producing this effect requires the presence of Mn in anamount more than or equal to 0.5%. However, if Mn is present in anamount more than 2.0%, grain boundary strength is degraded, and,therefore, low-temperature toughness is degraded. Accordingly, theamount of Mn is limited to a range of 0.5 to 2.0%. Note that the amountis preferably more than or equal to 0.8% and less than or equal to 1.5%.

P: 0.05% or Less

P tends to be segregated at grain boundaries and the like, whichdegrades the bonding strength of grains, which in turn degradestoughness. Accordingly, it is desirable that an amount of P be as low aspossible; a permissible amount of P is up to 0.05%. Hence, the amount ofP is limited to less than or equal to 0.05%.

S: 0.010% or Less

S tends to be segregated at grain boundaries and tends to form MnS,which is a non-metallic inclusion; consequently, ductility and toughnessare degraded. Accordingly, it is desirable that an amount of S be as lowas possible; a permissible amount of S is up to 0.010%. Hence, theamount of S is limited to less than or equal to 0.010%.

N: 0.0005 to 0.0080%

N combines with nitride-forming elements, such as Nb, Ti, and Al, toform nitrides, which pin austenite grains to inhibit the coarsening ofgrains during heating; therefore, N has an effect of refining themicrostructure. Producing the microstructure-refining effect requiresthe presence of N in an amount more than or equal to 0.0005%. On theother hand, if N is present in an amount more than 0.0080%, an amount ofdissolved N is increased, and, consequently, the toughness of the basemetal and a weld heat affected zone is degraded. Accordingly, the amountof N is limited to a range of 0.0005 to 0.0080%. Note that the amount ispreferably more than or equal to 0.0020% and less than or equal to0.0050%.

O: 0.0100% or Less

O (oxygen) increases an amount of non-metallic inclusions by formingoxides, such as alumina, which results in degradation of workability,for example, degradation of ductility. Accordingly, it is desirable thatan amount of O (oxygen) be as low as possible; a permissible amount of O(oxygen) is up to 0.0100%. Accordingly, the amount of O (oxygen) islimited to less than or equal to 0.0100%. Note that the amount ispreferably less than or equal to 0.0050%.

The basic composition described above includes the components describedabove. In addition to the basic composition described above, any of thefollowing optional elements may be selected and included: one or moreselected from Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%, Mo: 0.05 to 2.00%,W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.200%, Ti: 0.005 to0.100%, and B: 0.0005 to 0.0050% and/or one or more selected from Nd:0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and oneor more REMs: 0.0005 to 0.0050%.

Ni, Cr, Mo, W, Nb, V, Ti, and B are all elements that contribute toimproving hardenability. One or more of these may be selected asnecessary and included.

Ni: 0.05 to 2.00%

Ni is an element that has an effect of improving toughness in additionto improving hardenability. Producing these effects requires thepresence of Ni in an amount more than or equal to 0.05%. On the otherhand, if Ni is present in an amount more than 2.00%, a material cost isincreased, and, therefore, an economic advantage is reduced.Accordingly, in instances where Ni is to be present, it is preferablethat the amount of Ni be limited to a range of 0.05 to 2.00%. Note thatthe amount is more preferably more than or equal to 0.50% and less thanor equal to 1.50%.

Cr: 0.10 to 2.50%

Cr is an element that contributes to ensuring a strength by improvinghardenability. Producing this effect requires the presence of Cr in anamount more than or equal to 0.10%. On the other hand, if a large amountof Cr is present, that is, an amount more than 2.50%, weldability isdegraded. Accordingly, in cases where Cr is to be present, it ispreferable that the amount of Cr is limited to a range of 0.10 to 2.50%.Note that the amount is more preferably more than or equal to 0.50% andless than or equal to 1.50%.

Mo: 0.05 to 2.00%

Mo is an element that contributes to ensuring a strength by improvinghardenability. Producing this effect requires the presence of Mo in anamount more than or equal to 0.05%. On the other hand, if a large amountof Mo is present, that is, an amount more than 2.00%, a material cost isincreased, and, therefore, an economic advantage is reduced.Accordingly, in instances where Mo is to be present, it is preferablethat the amount of Mo be limited to a range of 0.05 to 2.00%. Note thatthe amount is more preferably more than or equal to 0.20% and less thanor equal to 1.50%.

W: 0.05 to 2.00%

W is an element that contributes to ensuring a strength by improvinghardenability. Producing this effect requires the presence of W in anamount more than or equal to 0.05%. On the other hand, if a large amountof W is present, that is, an amount more than 2.00%, weldability isdegraded. Accordingly, in instances where W is to be present, it ispreferable that the amount of W be limited to 0.05 to 2.00%. Note thatthe amount is more preferably more than or equal to 0.20% and less thanor equal to 1.50%.

Nb: 0.005 to 0.100%

Nb is an element that has an effect of improving hardenability and, inaddition, an effect of inhibiting the coarsening of grains duringheating as Nb is finely precipitated as carbonitrides to pin austenitegrains. These effects can be observed in cases in which Nb is present inan amount more than or equal to 0.005%. On the other hand, if Nb ispresent in an amount more than 0.100%, the toughness of a weld heataffected zone is degraded. Accordingly, in instances where Nb is to bepresent, it is preferable that the amount of Nb be limited to a range of0.005 to 0.100%. Note that the amount is more preferably more than orequal to 0.010% and less than or equal to 0.050%.

V: 0.005 to 0.200%

V is an element that has an effect of improving hardenability and, inaddition, an effect of inhibiting the coarsening of grains duringheating as V is finely precipitated as carbonitrides to pin austenitegrains. These effects can be observed in cases in which V is present inan amount more than or equal to 0.005%. On the other hand, if V ispresent in an amount more than 0.200%, the toughness of a weld heataffected zone is degraded. Accordingly, in instances where V is to bepresent, it is preferable that the amount of V be limited to a range of0.005 to 0.200%. Note that the amount is more preferably more than orequal to 0.010% and less than or equal to 0.150%.

Ti: 0.005 to 0.100%

Ti is an element that has an effect of improving hardenability and, inaddition, an effect of inhibiting the coarsening of grains duringheating as Ti is finely precipitated as carbonitrides to pin austenitegrains. These effects can be observed in cases in which Ti is present inan amount more than or equal to 0.005%. On the other hand, if Ti ispresent in an amount more than 0.100%, the toughness of a weld heataffected zone is degraded. Accordingly, in instances where Ti is to bepresent, it is preferable that the amount of Ti be limited to a range of0.005 to 0.100%. Note that the amount is more preferably more than orequal to 0.010% and less than or equal to 0.050%.

B: 0.0005 to 0.0050%

B is an element that contributes to improving hardenability even when Bis present in a small amount. Producing this effect requires thepresence of B in an amount more than or equal to 0.0005%. On the otherhand, if B is present in an amount more than 0.0050%, toughness isdegraded. Accordingly, in instances where B is to be present, it ispreferable that the amount of B be limited to a range of 0.0005 to0.0050%. Note that the amount is more preferably more than or equal to0.0010% and less than or equal to 0.0020%.

Furthermore, Nd, Ca, Mg, and REMs are all elements that contribute toimproving ductility, toughness, and hydrogen embrittlement resistance bycontrolling the morphology of inclusions. One or more of these may beselected as necessary and included.

Nd: 0.005 to 1.000%

Nd is an element that combines with S to form sulfide inclusions,thereby reducing an amount of grain boundary segregation of S and thuscontributing to improving toughness and hydrogen brittleness resistance.Producing this effect requires the presence of Nd in an amount more thanor equal to 0.005%. On the other hand, if Nd is present in an amountmore than 1.000%, the toughness of a weld heat affected zone isdegraded. Accordingly, in instances where Nd is to be present, it ispreferable that the amount of Nd be limited to a range of 0.005 to1.000%. Note that the amount is more preferably more than or equal to0.010% and less than or equal to 0.500%.

Ca: 0.0005 to 0.0050%

Ca has high affinity for S and forms CaS in place of MnS; CaS is aglobular sulfide inclusion, which is not easily elongated in rolling asopposed to MnS, which is a sulfide inclusion that is easily elongated inrolling. Accordingly, Ca is an element that contributes to controllingthe morphology of sulfide inclusions and has an effect of improvingductility and toughness. Producing this effect requires the presence ofCa in an amount more than or equal to 0.0005%. On the other hand, if Cais present in an amount more than 0.0050%, cleanliness is degraded, andductility, toughness, and the like are degraded. Accordingly, ininstances where Ca is to be present, it is preferable that the amount ofCa be limited to a range of 0.0005 to 0.0050%. Note that the amount ismore preferably more than or equal to 0.0010% and less than or equal to0.0020%.

Mg: 0.0005 to 0.0050%

Similar to Ca, Mg has high affinity for S and forms sulfide inclusions,thereby improving ductility and toughness. Producing this effectrequires the presence of Mg in an amount more than or equal to 0.0005%.On the other hand, if Mg is present in an amount more than 0.0050%,cleanliness is degraded. Accordingly, in instances where Mg is to bepresent, it is preferable that the amount of Mg be limited to a range of0.0005 to 0.0050%. Note that the amount is more preferably more than orequal to 0.0010% and less than or equal to 0.0020%.

One or More REMs: 0.0005 to 0.0050%

REMs are elements that form sulfide inclusions, such as REM(O, S), toreduce an amount of dissolved S at grain boundaries, therebycontributing to improving SR cracking resistance. Producing this effectrequires the presence of one or more REMs in an amount more than orequal to 0.0005%. On the other hand, if one or more REMs are present inan amount more than 0.0050%, large amounts of REM sulfide inclusionsaccumulate in a sedimentation zone during casting, and, consequently,material properties, such as ductility and toughness, are degraded.Accordingly, in instances where one or more REMs are to be present, itis preferable that the amount of one or more REMs be limited to a rangeof 0.0005 to 0.0050%. Note that the amount is more preferably more thanor equal to 0.0010% and less than or equal to 0.0020%. Note that as usedherein, the term “REM” is an abbreviation of “rare earth metal”.

The balance, other than the components described above, is Fe andincidental impurities.

Steel materials for a high-pressure hydrogen gas environment of thedisclosed embodiments are steel materials that have the compositiondescribed above and have a microstructure formed of a combination offerrite and pearlite or formed of lower bainite, martensite, temperedlower bainite, tempered martensite, or a combination of any of these.

Furthermore, the steel materials for a high-pressure hydrogen gasenvironment of the disclosed embodiments are steel materials that havethe composition described above and the microstructure described above,the steel materials have a high strength of 560 MPa or higher in termsof tensile strength, and a fracture toughness value K_(IH) exhibited bythe steel materials in a high-pressure hydrogen gas atmosphere is 40MPa·m^(1/2) or higher; therefore, the steel materials have excellenthydrogen embrittlement resistance.

Now, preferred methods of the disclosed embodiments for producing asteel material for a high-pressure hydrogen gas environment will bedescribed.

First, molten steel having the composition described above is producedin a common steel-making furnace, such as a converter or an electricfurnace, and the molten steel is subjected to a continuous castingprocess to form a cast steel having a predetermined shape, such as aslab, or the molten steel is subjected to an ingot casting process orthe like, in which a cast steel (steel ingot) is hot-rolled to form aworkpiece having a predetermined shape, such as a slab; accordingly, asteel starting material is formed.

Subsequently, the obtained steel starting material is loaded into aheating furnace. A heating temperature is Ac₃ transformation temperatureor higher. If the heating temperature is less than Ac₃ transformationtemperature, the material to be rolled has a high deformationresistance, which results in an excessive load on a rolling machine, andin addition, a partial untransformed constituent remains; consequently,the desired characteristics cannot be ensured even with subsequentprocessing. Note that the heating temperature is preferably 1100 to1300° C. If the heating temperature is less than 1100° C., thedeformation resistance is high, which results in an excessively highload on a rolling machine. On the other hand, if the heating temperatureis higher than 1300° C., coarsening of grains occurs, which results indegraded toughness.

Subsequently, the steel starting material, which has been heated to apredetermined temperature, is subjected to hot rolling to form a steelmaterial having a predetermined size and shape. As used herein, the term“steel material” encompasses sheets, plates, steel pipes, shaped steels,steel bars, and the like. Furthermore, as used herein, the term “hotrolling” is not meant to specify any particular rolling conditions; itis sufficient that the hot rolling can form a steel material having apredetermined size and shape. In instances where the steel material is aseamless steel pipe, the hot rolling is rolling that includes piercingrolling.

It is preferable that the steel material rolled to have a predeterminedsize and shape be processed in any of the following manners: the steelmaterial is allowed to cool to room temperature and, after having beencooled, subjected to a reheating-quenching-tempering process, in whichthe steel material is reheated, quenched, and tempered; after the hotrolling, the steel material is subjected to an accelerated coolingprocess; or after the hot rolling, the steel material is subjected to adirect quenching-tempering process.

Now, the accelerated cooling process, the direct quenching-temperingprocess, and the reheating-quenching-tempering process will be describedindividually.

Note that the temperature specified in the production conditions is atemperature of a middle portion of the steel material. In instanceswhere the steel material is a sheet, plate, steel pipe, or shapedsteels, the middle portion is a middle of a thickness (wall thickness),and in instances where the steel material is a steel bar, the middleportion is a middle in a radial direction. Since the vicinity of themiddle portion has a substantially uniform temperature history, thetemperature specified is not limited to the temperature of the exactmiddle.

Accelerated Cooling Process

The steel material rolled to have a predetermined size and shape isthereafter, without being cooled to room temperature, subjected to anaccelerated cooling process; in the accelerated cooling process, thesteel material is cooled from a cooling start temperature of (Ar₃transformation temperature—50° C.) or higher to a cooling stoptemperature of 600° C. or lower at a cooling rate of 1 to 200° C./s. Ifthe cooling start temperature is less than (Ar₃ transformationtemperature—50° C.), an amount of transformation of austenite isincreased before the start of the cooling, and, consequently, thecharacteristics that exist after the accelerated cooling are not thedesired ones. Accordingly, the cooling start temperature is limited to atemperature of (Ar₃ transformation temperature—50° C.) or higher.Furthermore, if the cooling rate for the accelerated cooling is lessthan 1° C./s, the cooling is too slow, and, consequently, the desiredcharacteristics cannot be ensured. On the other hand, in cases where atypical cooling method is used, the cooling rate does not exceed 200°C./s. Accordingly, the cooling rate for the accelerated cooling processis limited to the range of 1 to 200° C./s. Note that the cooling rate isan average cooling rate in a thickness (wall thickness) middle. Themeans for the cooling need not be particularly limited, and it ispreferable that water cooling, for example, be used. Furthermore, if thecooling stop temperature for the accelerated cooling is a hightemperature of higher than 600° C., a desired transformation is notaccomplished, and, therefore, the desired characteristics cannot beensured. Accordingly, the cooling stop temperature for the acceleratedcooling is limited to a temperature of 600° C. or lower.

Direct Quenching-Tempering Process

The steel starting material is heated to a temperature of the Ac₃transformation temperature or higher and hot-rolled to form a steelmaterial having a predetermined size and shape; thereafter, the steelmaterial is subjected to a quenching process, in which the steelmaterial is cooled from a temperature of (Ar₃ transformationtemperature—50° C.) or higher to a cooling stop temperature of 250° C.or lower at a cooling rate of 1 to 200° C./s; and thereafter, the steelmaterial is subjected to a tempering process, in which the steelmaterial is tempered at a tempering temperature of Ac₁ transformationtemperature or lower. If the heating temperature for the steel startingmaterial is less than the Ac₃ transformation temperature, a partialuntransformed constituent remains, and consequently, the microstructurethat exists after the hot rolling and the quenching-tempering is not adesired one. Accordingly, the heating temperature before the hot rollingis specified to be Ac₃ transformation temperature or higher.Furthermore, if the starting temperature for the quenching after the hotrolling is less than (Ar₃ transformation temperature—50° C.), the amountof transformation of austenite before the quenching is increased, andconsequently, the microstructure that exists after thequenching-tempering is not a desired one. Accordingly, after the hotrolling, the cooling is to be started at a temperature of (Ar₃transformation temperature—50° C.) or higher to carry out the quenching.For the quenching that starts at (Ar₃ transformation temperature—50° C.)or higher, the cooling rate is specified to be 1 to 200° C./s to obtaina desired microstructure. Note that the cooling rate is an averagecooling rate in a thickness middle. The means for the cooling need notbe particularly limited, and, for example, water cooling may be used.Furthermore, if the cooling rate for the quenching is less than 1° C./s,the cooling is too slow, and, consequently, the desired characteristicscannot be ensured. On the other hand, in cases where a typical coolingmethod is used, the cooling rate does not exceed 200° C./s. Furthermore,if the quenching is stopped at a temperature of higher than 250° C.,desired martensitic transformation and/or bainitic transformation arenot accomplished, and, consequently, the characteristics that existafter the tempering are not the desired ones. Accordingly, in thequenching process, the cooling is to be performed until a temperature of250° C. or lower is reached. After the quenching, the steel material istempered at a temperature of Ac₁ transformation temperature or lower. Ifthe tempering temperature is higher than Ac₁ transformation temperature,partial austenite transformation occurs, and, consequently, thecharacteristics that exist after the tempering are not the desired ones.

Reheating-Quenching-Tempering Process

The steel material rolled to have a predetermined size and shape is thencooled to room temperature, and subsequently, the steel material issubjected to a reheating-quenching-tempering process, in which the steelmaterial is heated at a quenching heating temperature of Ac₃transformation temperature or higher; thereafter, the steel material issubjected to a quenching process, in which the steel material is cooledfrom a quenching start temperature of (Ar₃ transformationtemperature—50° C.) or higher to a temperature of 250° C. or lower at acooling rate of 0.5 to 100° C./s; and subsequently, the steel materialis tempered at a temperature of Ac₁ transformation temperature or lower.

Note that it is preferable that the quenching process be carried out inthe following manner: water or oil, for example, is used as a coolingmedium, and the cooling medium is sprayed onto the steel material, whichis the cooling target heated to a high temperature, in a manner suchthat a cooling rate of 0.5 to 100° C./s is achieved, or the heated steelmaterial is immersed in a tank that holds the cooling medium. From thestandpoint of achieving uniform cooling, it is preferable that in a tankthat holds the cooling medium, the steel material having a predeterminedsize and shape be cooled by a jet stream of the cooling medium sprayedthereto while the steel material is rotated. Furthermore, regarding thetempering process, the steel material heated in a tempering heatingfurnace or the like may be allowed to cool in air or a protectiveatmosphere.

If the quenching heating temperature is less than Ac₃ transformationtemperature, a partial untransformed constituent remains, and,consequently, the characteristics that exist after thequenching-tempering are not the desired ones. Accordingly, the quenchingheating temperature is specified to be Ac₃ transformation temperature orhigher. Furthermore, if the quenching start temperature is less than(Ar₃ transformation temperature—50° C.), the transformation of theaustenite begins before the start of the quenching, and, consequently,the characteristics that exist after the quenching-tempering are not thedesired ones. Accordingly, the quenching start temperature is limited toa temperature of (Ar₃ transformation temperature—50° C.) or higher.Furthermore, the cooling rate for the quenching is limited to 0.5 to100° C./s to achieve the desired characteristics and prevent quenchcracking. If the quench cooling stop temperature is a high temperatureof higher than 250° C., a desired transformation (martensitictransformation or bainitic transformation) is not accomplished, and,consequently, the characteristics that exist after the tempering processare not the desired ones. Accordingly, the quench stop temperature islimited to a temperature of 250° C. or lower.

After the quenching process, a tempering process is performed in whichthe steel material is tempered by being heated to a temperingtemperature of Ac₁ transformation temperature or lower. If the temperingtemperature is higher than Ac₁ transformation temperature, partialaustenite transformation occurs, and, consequently, the characteristicsthat exist after the tempering process are not the desired ones.

Note that Ac₃ transformation temperature (° C.), Ar₃ transformationtemperature (° C.), and Ac₁ transformation temperature (° C.) describedabove, which are used herein, are temperatures calculated by using thefollowing equations.

Ac₃ (° C.)=854−180C+44Si−14Mn−17.8Ni−1.7Cr

Ar₃ (° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo

Ac₁ (° C.)=723−14Mn+22Si−14.4Ni+23.3Cr

Here, the chemical symbols represent a content (mass %) of the elementin the steel.

The steel material having excellent hydrogen embrittlement resistanceproduced by the production method described above is suitable for use insteel structures for hydrogen that are used in a high-pressure hydrogengas environment. Examples of the “steel structures for hydrogen” asreferred to herein include storage tanks (hydrogen storage tanks) thatare used in hydrogen stations and the like; and line pipes fortransportation of hydrogen gas (hydrogen line pipes).

As storage tanks that are used in hydrogen stations and the like, Type1, Type 2, and Type 3 are known. Type 1 is made exclusively of a steelmaterial, and Type 2 and Type 3 are made of a steel material and acarbon fiber reinforced plastic (CFRP) wound therearound. These typesare based on classifications regarding a structure of the container,which are described in, for example, various standards for compressednatural gas vehicle fuel containers, ISO 11439, ANSI/NGV, the ContainerSafety Rules-Exemplified Standard-Appendix-9 of the High Pressure GasSafety Act, and the like. Note that it is preferable that the storagetanks be produced, for example, by forming a steel material having thecomposition described above into a predetermined shape and subsequentlysubjecting the steel material to the reheating-quenching-temperingprocess. Note that the design pressure for the hydrogen to be stored ina storage tank is approximately 35 MPa or approximately 70 MPa.

Furthermore, as a line pipe for transportation of hydrogen, a seamlesssteel pipe, an electric resistance welded steel pipe, or a UOE-typesteel pipe is suitable. Note that it is preferable that the line pipe beformed as follows: a steel material having the composition describedabove is used as it is to form a line pipe (steel pipe), or a steelstarting material having the composition described above and subjectedto the accelerated cooling process described above or the directquenching process described above is used to form a steel pipe. Notethat, in line pipes, the design pressure for the hydrogen to be used isapproximately 10 MPa.

Now, the disclosed embodiments will be further described based onexamples.

EXAMPLES

Molten steel having a composition as shown in Table 1 was produced in aconverter and continuously cast to form cast steel (a slab, wallthickness: 250 mm). The obtained cast steel was heated and hot-rolled toform a steel plate (thickness: 38 mm), which was then cooled to roomtemperature. Subsequently, the steel plate was subjected to areheating-quenching-tempering process under the conditions shown inTable 2 (steel plates No. 1 to No. 16 and No. 21 to No. 23). Note thatthe quenching process was carried out by using water cooling or oilcooling.

Furthermore, the obtained cast steel was heated under the conditionshown in Table 2 and hot-rolled to form a steel plate having apredetermined thickness (38 mm), and thereafter, the steel plate wassubjected to an accelerated cooling process, which was performed underthe conditions shown in Table 2 (steel plates No. 17 and No. 18).

Furthermore, the obtained cast steel was heated under the conditionshown in Table 2 and hot-rolled to form a steel plate having apredetermined thickness (38 mm). Thereafter, the steel plate wassubjected to a direct quenching-tempering process, in which the steelplate was directly quenched under the conditions shown in Table 2 andwas subsequently tempered at the tempering temperature shown in Table 2(steel plates No. 19 and No. 20). Note that the temperature of the steelplate was measured by using a thermocouple inserted in a thicknessmiddle portion.

The reheating-quenching-tempering process was intended to simulate theproduction of a hydrogen storage tank, and the accelerated coolingprocess and the direct quenching process were both intended to simulatethe production of a hydrogen line pipe (steel pipe).

A tensile test, a fracture toughness test, and microstructureexamination were conducted on the obtained steel plates. The testmethods were as follows.

(1) Tensile Properties

A full-thickness tensile test piece was cut from the obtained steelplate in accordance with JIS Z 2201 (1980) such that a longitudinaldirection (tensile direction) of the test piece coincided with a rollingdirection, and a tensile test was conducted in accordance with thespecifications of JIS Z 2241 to measure the tensile strength.

(2) Fracture Toughness Test

A CT test piece (width: 50.8 mm) was cut from each of the obtained steelplates such that the load application direction was parallel to therolling direction. A fracture toughness test was conducted in ahigh-pressure hydrogen gas atmosphere in accordance with The JapanPressure container Research Council, Division of Materials Science andTechnology, Hydrogen Gas Embrittlement Technical Committee, Task Group V(1991). Accordingly, the fracture toughness value K_(IH) was determined.Note that the test was conducted in a high-pressure hydrogen gasatmosphere at room temperature (20±10° C.) and a pressure of 115 MPa ata constant displacement speed of 2.5 μm/min.

Note that, in some instances, a fracture toughness test in accordancewith the specifications of ASTM E399 or ASTM E1820 was also conducted toobtain fracture toughness values K_(IH). These fracture toughness valuesK_(IH) were not shown in Table 2 because these fracture toughness valuesK_(IH) were substantially equal to the fracture toughness values K_(IH)obtained in the fracture toughness test conducted in accordance with TheJapan Pressure container Research Council, Division of Materials Scienceand Technology, Hydrogen Gas Embrittlement Technical Committee, TaskGroup V (1991), with an error of not larger than 5%.

(3) Microstructure Examination

A test piece for microstructure examination was cut from a thicknessmiddle portion of the obtained steel plate, the test piece was polishedand etched (with a nital solution), and examination was conducted withan optical microscope (magnification: 200×). Accordingly, theconstituents were identified, and the fractions of the constituents werecalculated by the image analysis.

The results obtained are shown in Table 2.

TABLE 1 Steel Chemical components (mass %) No. C Si Mn P S Al N O Cu Ni,Cr, Mo, Nb, V, Ti, B Nd, W, Ca, Mg, REM Notes A 0.36 0.42 0.76 0.020.0031 0.051 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012Comparative Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010B 0.36 0.54 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.56 Ni: 0.32, Cr:1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti:0.012, B: 0.0010 C 0.36 1.02 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.56Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, exampleV: 0.041, Ti: 0.012, B: 0.0010 D 0.36 1.96 0.76 0.02 0.0031 0.051 0.00350.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb:0.023, example V: 0.041, Ti: 0.012, B: 0.0010 E 0.36 0.56 0.76 0.020.0031 0.051 0.0035 0.0033 0.42 Ni: 0.32, Cr: 1.06, Ca: 0.0012Comparative Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010F 0.36 0.56 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.53 Ni: 0.32, Cr:1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti:0.012, B: 0.0010 G 0.36 0.56 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.99Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, exampleV: 0.041, Ti: 0.012, B: 0.0010 H 0.36 0.56 0.76 0.02 0.0031 0.051 0.00350.0033 1.97 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb:0.023, example V: 0.041, Ti: 0.012, B: 0.0010 I 0.36 0.56 0.76 0.020.0031 0.008 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012Comparative Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010J 0.36 0.56 0.76 0.02 0.0031 0.52 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06,Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012,B: 0.0010 K 0.36 0.56 0.76 0.02 0.0031 0.97 0.0035 0.0033 0.56 Ni: 0.32,Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041,Ti: 0.012, B: 0.0010 L 0.36 0.56 0.76 0.02 0.0031 1.96 0.0035 0.00330.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023,example V: 0.041, Ti: 0.012, B: 0.0010 M 0.06 0.54 0.76 0.01 0.00310.051 0.0035 0.0033 0.53 Ni: 0.32, Cr: 1.06, Ca: 0.0012, Nd: 0.21,Conforming Mo: 1.08, Nb: 0.023, W: 0.15, Mg: 0.0006, example V: 0.041,Ti: 0.012, REM: 0.0005 B: 0.0010 N 0.06 0.54 0.76 0.01 0.0031 0.520.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012, Nd: 0.21, ConformingMo: 1.08, Nb: 0.023, W: 0.15, Mg: 0.0006, example V: 0.041, Ti: 0.012,REM: 0.0005 B: 0.0010 O 0.06 0.56 0.76 0.01 0.0031 0.52 0.0035 0.00330.53 Ni: 0.32, Cr: 1.06, Ca: 0.0012, Nd: 0.21, Conforming Mo: 1.08, Nb:0.023, W: 0.15, Mg: 0.0006, example V: 0.041, Ti: 0.012, REM: 0.0005 B:0.0010 P 0.06 0.54 0.76 0.01 0.0031 0.52 0.0035 0.0033 0.53 Ni: 0.32,Cr: 1.06, Ca: 0.0012, Nd: 0.21, Conforming Mo: 1.08, Nb: 0.023, W: 0.15,Mg: 0.0006, example V: 0.041, Ti: 0.012, REM: 0.0005 B: 0.0010 Q 0.481.02 1.82 0.01 0.0006 1.02 0.0035 0.0033 1.02 — — Conforming example R0.48 1.02 1.82 0.01 0.0006 1.02 0.0035 0.0033 1.02 Ni: 0.32, Cr: 1.06, —Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 S0.48 1.02 1.82 0.01 0.0006 1.02 0.0035 0.0033 1.02 — Ca: 0.0012Conforming example The underline indicates the value is outside therange of the disclosed embodiments.

TABLE 2 Test results Heating Cooling Fracture Steel TransformationThickness of Type of Heating Cooling start Cooling stop TemperingConstituent** and Tensile toughness plate Steel temperature (° C.) steelplate production temperature Means for temperature temperature Coolingrate temperature fraction thereof strength value K_(IH) No. No. Ac₃ Ar₃Ac₁ (mm) method* (° C.) cooling (° C.) (° C.) (° C./s) (° C.) (area %)(MPa) (MPa · m^(1/2)) Notes 1 A 790 607 742 38 RQT 920 Oil cooling 850200 10 635 TM 876 36 Comparative example 2 B 795 607 744 38 RQT 920 Oilcooling 850 200 10 635 TM 885 83 Example 3 C 816 607 755 38 RQT 920 Oilcooling 850 200 10 635 TM 902 97 Example 4 D 858 607 776 38 RQT 920 Oilcooling 850 200 10 635 TM 922 111 Example 5 E 796 609 745 38 RQT 920 Oilcooling 850 200 10 635 TM 876 36 Comparative example 6 F 796 607 745 38RQT 920 Oil cooling 850 200 10 635 TM 888 81 Example 7 G 796 598 745 38RQT 920 Oil cooling 850 200 10 635 TM 915 95 Example 8 H 796 578 745 38RQT 920 Oil cooling 850 200 10 635 TM 924 97 Example 9 I 796 607 745 38RQT 920 Oil cooling 850 200 10 635 TM 876 36 Comparative example 10 J796 607 745 38 RQT 920 Oil cooling 850 200 10 635 TM 886 77 Example 11 K796 607 745 38 RQT 920 Oil cooling 850 200 10 635 TM 905 89 Example 12 L796 607 745 38 RQT 920 Oil cooling 850 200 10 635 TM 923 91 Example 13 M849 700 744 38 RQT 920 Water 850 200 20 680 TM 1031 122 Example cooling14 N 849 700 744 38 RQT 920 Water 850 200 20 680 TM 1025 117 Examplecooling 15 O 850 700 745 38 RQT 920 Water 850 200 20 680 TM 1045 112Example cooling 16 P 849 700 744 38 RQT 920 Water 850 200 20 680 TM 1089155 Example cooling 17 M 849 700 744 38 AC 1100 Water 850 500 25 — F + P566 121 Example cooling 18 N 849 700 744 38 AC 1100 Water 850 500 25 —F + P 561 119 Example cooling 19 O 849 700 744 38 DQT 1100 Water 850 20020 680 TM 1051 110 Example cooling 20 P 850 700 745 38 DQT 1100 Water850 200 20 680 TM 1097 157 Example cooling 21 Q 787 595 720 38 RQT 920Oil cooling 850 200 10 500 TB 623 105 Example 22 R 780 475 740 38 RQT920 Oil cooling 850 200 10 500 TB + TM 855 81 Example 23 S 787 595 72038 RQT 920 Oil cooling 850 200 10 500 TB 631 102 Example*Reheating-quenching-tempering process: RQT, Accelerated coolingprocess: AC, Direct quenching-tempering process: DQT **TM: Temperedmartensite, TB: Tempered lower bainite, F: Ferrite, P: Pearlite, B:Lower bainite, M: Martensite The underline indicates the value isoutside the range of the disclosed embodiments.

In all of the Examples, the fracture toughness value K_(IH) exhibited ina high-pressure hydrogen gas atmosphere at 115 MPa was 40 MPa·m^(1/2) orhigher, and, therefore, it can be said that excellent hydrogenembrittlement resistance was achieved. In contrast, in ComparativeExamples, which had a composition that is outside the range of thedisclosed embodiments, the fracture toughness value K_(IH) exhibited ina high-pressure hydrogen gas atmosphere was less than 40 MPa·m^(1/2),which indicated a low hydrogen embrittlement resistance. Note that inall of the Examples, a high strength of 560 MPa or higher in terms oftensile strength was achieved.

Hence, it was confirmed that the disclosed embodiments enable theproduction of products (steel structures for hydrogen) having excellenthydrogen embrittlement resistance.

1. A steel material for a high-pressure hydrogen gas environment, thesteel material having a chemical composition comprising, by mass %: C:0.04 to 0.50%; Si: 0.5 to 2.0%; Mn: 0.5 to 2.0%; P: 0.05% or less; S:0.010% or less; N: 0.0005 to 0.0080%; Al: 0.010% to 2.0%; O: 0.0100% orless; Cu: 0.5 to 2.0%; and the balance being Fe and incidentalimpurities, wherein the steel material has a tensile strength of 560 MPaor higher, and a fracture toughness value K_(IH) exhibited by the steelmaterial in a high-pressure hydrogen gas atmosphere is 40 MPa·m^(1/2) orhigher.
 2. The steel material for a high-pressure hydrogen gasenvironment according to claim 1, wherein the chemical compositionfurther comprises, by mass %, Al: 0.5 to 2.0%.
 3. The steel material fora high-pressure hydrogen gas environment according to claim 1, whereinthe chemical composition further comprises at least one group selectedfrom the following groups: Group A: at least one element selected fromthe group consisting of, by mass %, Ni: 0.05 to 2.00%, Cr: 0.10 to2.50%, Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V:0.005 to 0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050%, andGroup B: at least one element selected from the group consisting of, bymass %, Nd: 0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to0.0050%, and REM: 0.0005 to 0.0050%.
 4. (canceled)
 5. A steel structurefor a high-pressure hydrogen gas environment, the steel structurecomprising the steel material according to claim
 1. 6. The steelstructure for a high-pressure hydrogen gas environment according toclaim 5, wherein the steel structure is a storage tank or a line pipe.7. A method for producing a steel material for a high-pressure hydrogengas environment according to claim 1, the method comprising: heating asteel starting material having the chemical composition to a temperatureof Ac₃ transformation temperature or higher and then hot-rolling thesteel starting material to form the steel material having apredetermined shape; and thereafter subjecting the steel material to anaccelerated cooling process, in which the steel material is cooled froma temperature of (Ar₃ transformation temperature—50° C.) or higher to acooling stop temperature of 600° C. or lower at a cooling rate in arange of 1 to 200° C./s.
 8. A method for producing a steel material fora high-pressure hydrogen gas environment according to claim 1, themethod comprising: heating a steel starting material having the chemicalcomposition to a temperature of Ac₃ transformation temperature or higherand then hot-rolling the steel starting material to form the steelmaterial having a predetermined shape; thereafter subjecting the steelmaterial to a direct quenching-tempering process, in which the steelmaterial is cooled from a temperature of (Ar₃ transformationtemperature—50° C.) or higher to a cooling stop temperature of 250° C.or lower at a cooling rate in a range of 1 to 200° C./s; and temperingthe steel material at a temperature of Ac₁ transformation temperature orlower.
 9. A method for producing a steel material for a high-pressurehydrogen gas environment according to claim 1, the method comprising:subjecting the steel material having the chemical composition and beingformed to have a predetermined shape, to a reheating-quenching-temperingprocess, in which the steel material is heated to a temperature of Ac₃transformation temperature or higher; subsequently subjecting the steelmaterial to water quenching or oil quenching; and tempering the steelmaterial at a temperature of Ac₁ transformation temperature or lower.10. The method for producing a steel material for a high-pressurehydrogen gas environment according to claim 9, wherein the chemicalcomposition further comprises, by mass %, Al: 0.5 to 2.0%. 11-12.(canceled)
 13. The steel material for a high-pressure hydrogen gasenvironment according to claim 2, wherein the chemical compositionfurther comprises at least one group selected from the following groups:Group A: at least one element selected from the group consisting of, bymass %, Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%, Mo: 0.05 to 2.00%, W: 0.05to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.200%, Ti: 0.005 to 0.100%,and B: 0.0005 to 0.0050%, and Group B: at least one element selectedfrom the group consisting of, by mass %, Nd: 0.005 to 1.000%, Ca: 0.0005to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0050%.
 14. Asteel structure for a high-pressure hydrogen gas environment, the steelstructure comprising the steel material according to claim
 2. 15. Asteel structure for a high-pressure hydrogen gas environment, the steelstructure comprising the steel material according to claim
 3. 16. Asteel structure for a high-pressure hydrogen gas environment, the steelstructure comprising the steel material according to claim
 13. 17. Thesteel structure for a high-pressure hydrogen gas environment accordingto claim 14, wherein the steel structure is a storage tank or a linepipe.
 18. The steel structure for a high-pressure hydrogen gasenvironment according to claim 15, wherein the steel structure is astorage tank or a line pipe.
 19. The steel structure for a high-pressurehydrogen gas environment according to claim 16, wherein the steelstructure is a storage tank or a line pipe.
 20. The method for producinga steel material for a high-pressure hydrogen gas environment accordingto claim 7, wherein the chemical composition further comprises, by mass%, Al: 0.5 to 2.0%.
 21. The method for producing a steel material for ahigh-pressure hydrogen gas environment according to claim 7, wherein thechemical composition further comprises at least one group selected fromthe following groups: Group A: at least one element selected from thegroup consisting of, by mass %, Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%,Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050%, and Group B: atleast one element selected from the group consisting of, by mass %, Nd:0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM:0.0005 to 0.0050%.
 22. The method for producing a steel material for ahigh-pressure hydrogen gas environment according to claim 20, whereinthe chemical composition further comprises at least one group selectedfrom the following groups: Group A: at least one element selected fromthe group consisting of, by mass %, Ni: 0.05 to 2.00%, Cr: 0.10 to2.50%, Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V:0.005 to 0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050%, andGroup B: at least one element selected from the group consisting of, bymass %, Nd: 0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to0.0050%, and REM: 0.0005 to 0.0050%.
 23. The method for producing asteel material for a high-pressure hydrogen gas environment according toclaim 8, wherein the chemical composition further comprises, by mass %,Al: 0.5 to 2.0%.